style: improve altcha widget integration and button alignment

- Style altcha widget to match theme using CSS variables (border, background, colors, dark mode)
- Make buttons in .btn-group stretch full width to align with inputs
- Center altcha widget with max-width: 100%

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

static/style.css

@@ -272,6 +272,10 @@ body {
   margin-top: 1.5rem;
 }
 
+.btn-group .btn {
+  flex: 1;
+}
+
 /* ============================================================
    Message page
    ============================================================ */
@@ -290,6 +294,76 @@ body {
    ============================================================ */
 .form-field altcha-widget {
   display: block;
+  max-width: 100% !important;
+  margin: 0 auto;
+}
+
+.form-field altcha-widget .altcha {
+  background: var(--input-bg);
+  border: 1px solid var(--border);
+  border-radius: 8px;
+  color: var(--text);
+  font-family: inherit;
+  font-size: 0.9375rem;
+  padding: 0;
+  overflow: hidden;
+  max-width: 100% !important;
+}
+
+.form-field altcha-widget .altcha-main {
+  display: flex;
+  align-items: center;
+  gap: 0.75rem;
+  padding: 0.625rem 0.875rem;
+}
+
+.form-field altcha-widget .altcha-checkbox {
+  flex-shrink: 0;
+}
+
+.form-field altcha-widget .altcha-checkbox input[type="checkbox"] {
+  width: 1.125rem;
+  height: 1.125rem;
+  accent-color: var(--btn-primary-bg);
+  cursor: pointer;
+  margin: 0;
+}
+
+.form-field altcha-widget .altcha-label {
+  flex: 1;
+}
+
+.form-field altcha-widget .altcha-label label {
+  cursor: pointer;
+  color: var(--text);
+}
+
+.form-field altcha-widget .altcha-logo {
+  color: var(--text-muted);
+  display: flex;
+  align-items: center;
+  transition: color .15s;
+}
+
+.form-field altcha-widget .altcha-logo:hover {
+  color: var(--text);
+}
+
+.form-field altcha-widget .altcha-footer {
+  font-size: 0.75rem;
+  color: var(--text-muted);
+  padding: 0.35rem 0.875rem;
+  border-top: 1px solid var(--border);
+  background: var(--btn-secondary-bg);
+}
+
+.form-field altcha-widget .altcha-footer a {
+  color: var(--btn-primary-bg);
+  text-decoration: none;
+}
+
+.form-field altcha-widget .altcha-footer a:hover {
+  text-decoration: underline;
 }
 
 /* ============================================================